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I have a 2004 Freestar with 240000 kilometers on it. I have had problem with the engine surging at about 1100 rpm or high idle. No check engine code is logged, I put it on a Ford computer and there were no results. After I cleaned the MAFS and IAC valve the vehicle ran good for a week or two. I found a loose wire on the IAC valve and fixed that, the engine ran well again for a while but then the IAC Valve would start cycling when the engine was hot. I thought I had it so I changed the IAC valve. Started the car, idles perfectly and revs up but as soon as I put it in gear the IAC valve starts to cycle much worse than the old one so the valve itself is working fine and I don't think that is the problem.

Does anyone know which sensors are feeding the computer so that it is sending a message to the IAC to make it cycle like that?

Bevan
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Dave Hodgson
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    it's based purely on engine RPM and ECT. does the car misfire? do you have access to a scantool to check fuel trims? Id suspect a vacuum leak or misfire. – Ben Apr 13 '16 at 21:01
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    @Ben - Didn't the Freestar have the issue with the bolts holding the lower plenum onto the block wearing out and causing a vacuum leak? I don't remember if it is the Freestar or the Windstar which had this issue. There is a fix kit for it. – Pᴀᴜʟsᴛᴇʀ2 Apr 13 '16 at 21:21
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    @Pᴀᴜʟsᴛᴇʀ2 It was the Windstar with the 3.8 IIRC The rubber material on the bolts is what went bad. – Ben Apr 13 '16 at 21:36
  • @Ben - That sounds right. – Pᴀᴜʟsᴛᴇʀ2 Apr 13 '16 at 22:29

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From my research on surging idle..A possible solution-TPS(Throttle Position Sensor) connector. I read on a diagnostic webpage that this guy disconnected and then reconnected it and the problem was solved for 3 weeks. In theory it makes sense when the ECM loses the input from the TPS sensor it falls back on a default setting until the TPS is reacquired. Maybe the ECM takes data from other sensors. If I were to speculate that the throttle position sensor is a variable resistor with a speck of dirt on the resistive element that would cause a lack of signal intermittently.

Old_Fossil
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